The aforementioned V. bonariensis sort of coming up wherever it wants to. Doesn't work. However, I think a long line of it in the peony bed such that it forms a purple hedge in late summer might work very well.

Phlox paniculata "David" in the back of the perennial bed behind all the giant, wild-type pink phlox. Can't see "David" well. Doesn't work. Must move it to in front of the pink phlox.

Oriental poppies in the middle of the perennial bed. Even when the foliage is brand new, it looks somewhat dry and unattractive. Once the poppies have bloomed (and they are spectacular) the plants go dormant for the summer, and I am left with, first, clumps of dying foliage and, second, a huge hole in the border. Doesn't work. Planning on moving the poppies to the back of the bed, where they'll be the tallest plants in June by a longshot. And easier to cover up with something else once the blooms are gone.